On Ammonia and Astrobiology

Jonathan Lunine of the Lunar and Planetary Laboratory at the University of ArizonaImage Credit: space.com

Jonathan Lunine, a professor of planetary science and physics at the University of Arizona’s Lunar and Planetary Laboratory in Tucson, Arizona, is also an interdisciplinary scientist on the Cassini/Huygens mission. Lunine presented a lecture entitled “Titan: A Personal View after Cassini’s first six months in Saturn orbit” at a NASA Director’s Seminar on January 24, 2005.

This edited transcript of the Director’s Seminar is Part 4 of a 4-part series.

“The presence of features seen in Cassini’s radar data would suggest that ammonia is at work on Titan, powering cryovolcanism.

Cryovolcanism – water-ice volcanism – requires a liquid layer in the interior of Titan. It also requires an agent to lower the melting point density and mobility of liquid water. Ammonia does all these very well.

Icy pebbles on Titan. Click image for larger view. Credit: ESA

There are three things about liquid water that are bad as a volcanic fluid. First, it only melts at 273 degrees Kelvin, which is a rather high temperature for the interior of Titan to be reaching after accretion. Second, liquid water is denser than water ice, so it’s hard to get it out onto the surface. And third, liquid water is very runny. It doesn’t produce features like the one we see in radar. But if you add ammonia to liquid water, at the right temperature you get a material with the mobility equivalent to that of basalt.

Ammonia not only reduces the mobility of the water, it also lowers the melting point by 100 degrees Celsius. It’s very easy to get a liquid ammonia water layer in Titan’s interior today; there’s no problem with that energetically. Furthermore, ammonia lowers the density of liquid water so that it’s neutrally buoyant relative to water ice.

One thing that Titan could not have done during its history is to have had a liquid layer that then froze over. During the freezing process of this layer in the interior, the tidal dissipation rate goes way up. This tidal dissipation would have reduced the eccentricity of Titan’s orbit, making it more circularized than it is.

Titan may never have had a liquid layer in its interior, but that’s hard to countenance, even for a pure water ice object, because accretion would melt the water, and then you get a re-freezing. The simplest explanation of why Titan still has a non-zero eccentricity is that the liquid layer in its interior has never frozen. The only way for it never to have been frozen is to have ammonia that allows for that low melting point solution.

The Cassini orbiter ion neutral mass spectrometer and the Huygens gas chromatograph mass spectrometer both sampled Titan’s atmosphere, and found there is essentially no non-radiogenic argon. There is argon-40 on Titan, and it is a radiogenic decay product of potassium. It implies that there has been outgassing from the interior of Titan. But there is no argon-36 or argon-38 to the level of 10 parts per million.

Argon is abundant in the solar system – it’s 10 percent the abundance of nitrogen in solar composition. So you would expect that Titan’s atmosphere would have at least 1 percent if not 10 percent argon. Instead, it has four orders of magnitude less than that. The most natural explanation is that the nitrogen that helped form Titan was a much less volatile form, which was easily attached to the ice, in a warm environment, which excluded argon. And that candidate is ammonia.

So ammonia’s the magic material. Now, if you have all these hydrocarbons and nitriles on the surface of Titan, and occasional contact of these with liquid water and liquid ammonia, by volcanism or impact, then chemistry possible during that time might include the production of amino acids, sugars, HCN, purines, peremidines and so on. That makes Titan an interesting object from the point of view of astrobiology.

Saturn rings in shadow. Image Credit: NASA/JPL

In the lab, tholins have been hydrolized and they do produce amino acids. But how far has organic chemistry gone on the surface of Titan? If indeed material is raining out from the stratosphere, then what happens to this material at the surface? To what extent is any chemistry on the surface at all relevant to the prebiotic chemistry that presumably occurred on the Earth prior to the time life began?

Cassini was not really instrumented to answer these questions about Titan, but they’re excellent questions for a follow-up mission. The answers are dependent on whether Cassini finds abundant organics on Titan’s surface. While the jury is still out on that, it’s looking better and better from the Huygens data and the orbiter data so far.

People have compared the reducing atmosphere of Titan with the classic Miller-Urey experiment, which used water and ammonia with methane to make all sorts of prebiotic molecules. But on Titan the water and ammonia are frozen out, except when you have cryovolcanism.

The Miller-Urey experiment used electric sparks to simulate lightning discharges. So far, there is no evidence for electrical discharges in Titan’s atmosphere. The atmospheric structure instrument on Huygens did have a lightning detector. It detected one pulse, but it was right at the moment that the main parachute pyros were fired, and that was probably the source of the pulse.

The energy available for weather on Titan is so small that the amount of lightning that could be sustained at any given time is very small, though it could be playing a role in the long term.

The next step beyond Cassini/Huygens, I think, is to go back to Titan with a mobile platform that can sample some of this organic stuff on the surface, and tell us whether any of it includes molecules of prebiotic interest.